Sintering Environment: Sintered silver joint is a porous silver that bonds semiconductor die to the substrate. One of the advantages of sintered silver joint is that nanoparticles can be sintered at temperature as low as 150C. To prevent oxidation of substrate, inert atmosphere is preferred environment for Ag paste. Current Ag paste formulation aims to sinter on substrates that are prone to oxidation in the ambient environment by including endothermic reducing Ag compounds in their formulations. Figure shows the densification of nano-Ag paste under different environment; (a) N2 ; (b) 1% O2/N2; (c) 4% H2N2. 1% O2/N2 sintered nano-Ag is denser than pure N2 and 4%H2/N2 sintered nano-Ag.

Nano Silver (Ag) Paste: Sintered silver joint is a porous silver that bonds semiconductor die to the substrate. One of the advantages of sintered silver joint is that nanoparticles can be sintered at temperature as low as 150C. The increase in surface area and curvature of the Ag nanoparticles provide the driving force for sintering at lower temperature and lower pressure. Figure shows the contribution of surface curvature of the nano sized grains and applied external pressure in densifying the materials. The intersection of these two parameters (i.e., curvature and pressure) depends on different materials system and formulations; the influence of surface curvature dominates the densification behavior below this grain size and vice versa for grains larger than this critical dimension.

The phase change or decomposition of polymers has been used in a variety of ways to produce air cavities and structures with ultralow dielectric constant, mechanical compliance or releasable layers. The use of air cavities in electronic devices lowers the energy dissipated and improves the interconnect speed. Figure shows mechanically compliant/compressible interconnects including the compressive regions where the embedded air cavity is visible. The addition of embedded air gaps into the interconnect provides vertical compliance needed for wafer level testing and mating to nonplanar boards. The use of compliant polymer encapsulating the air gap forms a structure capable of elastically deforming in all three directions.

Optical interconnects are a potential solution to some of the interconnect bottlenecks, especially clock distribution. Air cladding can be an enabling technology for guided wave optical interconnects. Figure shows a series of waveguides with air-cladding fabricated in Silicon so that optical waveguides could be routed like wires with sharp vertical and horizontal bends.

Thin film materials have many advantages in terms of properties, applications and economic considerations. Stretchable thin films have been shown to have promising applications in optical gratings, precision metrology and stretchable electronics. Figure shows a Silicon based integrated circuit that is stretchable, twistable and foldable. Bending of circuits to an extreme folded state is not possible by conventional flexible electronics. Stretchable electronics opens up many new opportunities and possibilities in design and application as compared to rigid electronics – for example in the biomedical area, energy harvesting, energy storage and wearable electronics.

Figure shows use in biomedical applications which is not possible with standard rigid electronics - an electronic eye camera with dynamically tunable zoom, eye cameras inspired by anthropod eyes, device integrated on rabbit heart, multifunctional balloon catheter integrated with sensors, actuators and other components. These systems were shown to provide very stable electronic performance.

Thermal Interface Materials (TIMs) are used to attach a heat spreader or heat sink to the backside of an electronic device. The thermal performance of the TIMs is critical for removal of the heat from the chip to the ambient as well as reliable operation of the devices, especially in defense applications where failure rate is often higher than in commercial equipment. The team from Georgia Institute of Technology, Purdue University and Raytheon have developed nTIM’s based on vertically aligned carbon nano tubes (CNTs). This gave 3X better thermal performance as compared to commercially available TIMs resulting in 10% increase in efficiency and 10X increase in reliability.

Thermal Interface Materials (TIMs) are used to attach a heat spreader or heat sink to the backside of an electronic device. The thermal performance of the TIMs is critical for removal of the heat from the chip to the ambient as well as reliable operation of the devices, especially in defense applications where failure rate is often higher than in commercial equipment. The team from General Electric (GE) Global Research have developed nTIM’s based on technique that forms metal nanosprings. Nanosprings are 100X more compliant than solders, so the thermal stresses are carried by the nanosprings rather than the solders, enabling thinner solder layers. This gave better thermal performance as compared to commercially available TIMs resulting in significant increase in efficiency and reliability.

TIMs are used to attach a heat spreader or heat sink to the backside of an electronic device. The thermal performance of the TIMs is critical for removal of the heat from the chip to the ambient as well as reliable operation of the devices, especially in defense applications where failure rate is often higher than in commercial equipment. The Carnegie Mellon University (CMU) approach for next generation TIM to achieve 10X improvement in thermal performance is based on silver and copper nanowires with embedded metal nanoparticles. The copper and silver nanowires give mechanical compliance which is 3X better than bulk copper or silver.

Two Phase Cooling Technologies: Stacking of multiple functional dice promises significant performance advantages for the next generation of computing and communication systems. However, increased power densities per chip footprint area results in challenging thermal problems. Technical challenges remain when designing systems to address the largest expected heat fluxes, which may be in excess of 1kW/cm2, with local hotspots exceeding 5kW/cm2. Two phase cooling has been identified as a potential solution for cooling of such modules. A key challenge to implementation of two phase cooling in 3D systems is the inevitable non-uniformity of power consumption within individual tiers, which gives rise to localized hotspots. Hotspots are problematic in two phase cooling systems, as the local variation in heat flux can cause flow oscillations and flow by-pass with reduction in the local heat transfer coefficient (a) Typical two phase force heat sink with no hotspot cooler, (b) “Fluid to fluid, spot to spreader” heat sink with separate background and hotspot cooler.

3D Stackable Evaporative Cooler (3D STAECOOL) Heat Sink: STAECOOL is inspired by the F2/S2 (Fluid to fluid, spot to spreader) hybrid heat-sink design. The thermal design seeks to achieve removal of very large (in excess of 500 W/cm2) background heat fluxes over a large 1cm1cm chip area, as well as extreme (in excess of 2 kW/cm2) hotspot heat fluxes over small 200 μm x 200 μm areas using two phase flow. This design combines a micro pin–fin heat sink for background cooling and localized, ultrathin micro gaps for hotspot cooling.